Low-frequency correction of image amplifiers



y 19.41 K. SCHLESINGER 2,248,550

LOW-FREQUENCY CORRECTION OF IMAGE AMPLIFIERS Filed Nov, 9, 1957 hie/Mar:

Patented July 8, 1941 LOW-FREQUENCY CORRECTION OF HYIAGE AMPLIFIERS Kurt Schlesinger, Berlin, Germany assignor, by mesne assignmenta to Loewe Radio, Inc., a cor- .poration of New York Application November 9, 1937, Serial No. 173,695

-" In Germany November 14, 1936 V 3Claims. (01. 179-171) The present invention relates to television receivers and more particularly to video signal arne plifiers the anode current of which is taken' from;

a storage condenser charged by a rectifier via a steadying resistance.

It is known that in this case'amplifiers with R. C.-coupling exhibit an increased amplification of lowfrequenciesI This efiectyhas even been utilised sometimes for compensating the dropin amplification of low'frequencies otherwise caused by capacitative coupling. The applicant ihas' found, however, that in the'caseof icorrectly dimensioned condenser coupling thereis no difliculty in the transmissionof the low video frequencies, but that the said increasein amplification owing to the stea'dying circuitleads to troublesome interferencesJ The amplifier gets .super-sensitive in respect of frequencies below 50 cycles and responds ,to verysmall impulses, for example to disturbances comingfrom. the mains, whereby there is causeda continuous .fiashing of the light intensity of the image. 'A more serious fault is that the described fault leads to detrimental image distortions, .viz., to incorrect light intensity values over large areas of the image, which is explained more concisely later on. The increase in amplification takes place in each single stage of the amplifier, and these-errors are cumulative.- The applicant has recognized that these disturbances arising in television may be compensated byuse of ajspecial circuit-system arranged in the cathode line of the R. C.-coupled amplifier so that the anode current is passing through this special circuit, consisting of a parallel connection of a resistance and a condenser.

Further objects of the invention may be best understood by reference to the following description taken in connection with the accompanying drawing, in which Figs. 1 and 6 are diagrams showing the connection of an amplifier according to theinvention, whilst Figs. 2 to 5 are graphs of grid and anode voltages.

Fig. 1 shows an amplifying stage consisting of a pentode l5 with anode potential source (mains supply) 8, steadying means (resistance 2 and storage condenser 3) and anode resistance I.

The input potential source 4 is coupled to the grid of the pentode via condenser 5 and resistance [2. The output device 6 is connected to the anode via a coupling condenser 1.

In the following the resistance l, is designawd R1, the capacity of condenser 3 C3; V is the amplification factor, S the steepness of the characteristic of the pentode. I f:

Upon considering Fig. 1 it will be clear that if the correcting circuit 9/) accordingjto the invention is missing the condenser 3 no longer performs short-circuiting, so that in respect, of very. low frequencies the totalof the resistances fl and 2 eases coupling resistance, so thatntlieampllfication increases from V1=S-B 1 to Vz=j(R +R2) The rateat which thisincrease takesplace possesses the time constant Ta=,Cz-Rz.

If at 4 apotential 6g of rectangular .forni (Fig;

2) is applied to the grid a potential ea (Fig.3) is obtained at 6, the values 2| and 22 corresponding to the amplification factors V1 and V2.

The applicant has found that it is possible to eliminate this distortion by inserting in the, lead to the cathode a correcting circuit, which consists of condenser 9shunted by resistance In.

The emission current forms after some time at I the, cathode a bias s'RlO'eg which is subtracted y from the input potential e at 4. The result is a corrective effective grid control potential as] shown in Fig. 4 which drops in such fas'hionthat the desired correct anode potential wave ea, ac,-

cording to Fig. 5 is obtained as a resultof the two.

effects in the cathode andanode circuits.

The theoretical calculation results in the under the effect of the normal anode current.

It may accordingly occur that the tube having this bias determined by R10 does not operate satisfactorily. In this case an additional bias ll must be supplied to the grid by way of the high resistance I2.

It has been found, however, that a still better correction is obtained by a connection according to Fig. 6. A resistance I6 is put in series to condenser 9 and, in order to obtain sufficient earthing of the cathode for high and medium frequencies an additional condenser I1 is provided in parallel to both. The proper value of resistance I6 is preferably ascertained by oscillographic observation of the output potential ea at 6 which is obtained from a rectangular grid input potential e; of approximately 25 cycles. Consideration shows that the filter condenser 3 is dynamically charged bytheseries resistance I and thus effects a phase difierence, whilst the condenser 8 if I6 is not provided is directly fed by anode current.

Values ascertained in practice in respect of acorrecting circuit are, for example, the following:

and, for a twofold amplification:

R1=1,000 ohms, R2=10,000 ohms, C3=20 mf.

It isclear that as many stages of this kind as desired, corrected with respectto the rectangular wave; can be connected in cascade, and that a multistage image amplifier is then also corrected inits entirety. A condition in this connection is that-.thecoupling elements 5/ lltransmit the low tones in question Without drop. For television purposes this condition even in the case of 25 image changes is completely fulfilled if for example Cs =0.5' mfi, R12=0.3 megohms.

I cl'aim: 7

ii A resistance-capacity coupled amplifier arrangement for television purposes comprising an amplifier. tube having at least a cathode, anode and control grid, an input and output circuit, a power supply circuit, an anode-resistance and steadying meansconsisting of a'steadying con-.

to R/V, said first correcting condenser having a value amounting to 0 times V, R being the steadying resistance, V being the normal amplification and C being the steadying condenser, said shunting condenser being so dimensioned as to short-circuit the middle and high frequencies.

2. A resistance. capacity coupled amplifier circuit arrangement for television purposes consisting of a plurality of single amplifier stages, each of said stages comprising an amplifier tube having atleast a cathode, anode and control grid, an input and output circuit, a power supply circuit, an anode-resistance and steadying means consisting of 'a steadying condenser and a steadying resistance in said output circuit, a compensating circuit connection for compensating the increase in amplification of the low frequencies effected by said steadying means, said compensating circuit connection being arranged within the cathode line of said tube carrying the anode current and consisting of a correcting-resistance to which on the one sidea-correcting condenser with ascrially connected resistanceand on the other side a shunting condenser is. connected in parallel, said,

correcting resistance having a resistance value amounting to. R/V, said correctingcondenser having a value amounting to C times V, Rbeing the steadying, resistance, V being the. normal amplification and C being the steadyingcondenser, said shunting condenser. being so'dimensioned' as to short-circuit the middle andhigh frequencies.

3. Ina wide bandamplifier, a thermionic tube.

having at least acathodeacontrol electrode, and an output electrode, means to apply input signals between the control. electrode and a predetermined point of fixedlpotential, means including a serially connected load-impedance and a decoupling resistor connected. to the. output. electrode for maintaining the output electrode. positive relative to saidpoint of fixed potential, a decoupling condenser connected between the junction of said load impedance and the decoupling resistor and the point of fixed potential, and a. parallelly' connected condenser and resistor connected between the cathode and the said point of fixed potential, said last'named parallelly connected resistor and condenser havinga time constant substantially equal to that of the decoupling resistor andcondenser, whereby low frequency components of applied signals are undistorte'dlyamplified;

KURT SCHLESINGER. 

